Environmental hydro-refugia demonstrated by vegetation vigour in the Okavango Delta, Botswana

Abstract

Climate shifts at decadal scales can have environmental consequences, and therefore, identifying areas that act as environmental refugia is valuable in understanding future climate variability. Here we illustrate how, given appropriate geohydrology, a rift basin and its catchment can buffer vegetation response to climate signals on decadal time-scales, therefore exerting strong local environmental control. We use time-series data derived from Normalised Difference Vegetation Index (NDVI) residuals that record vegetation vigour, extracted from a decadal span of MODIS images, to demonstrate hydrogeological buffering. While this has been described previously it has never been demonstrated via remote sensing and results in relative stability in vegetation vigour inside the delta, compared to that outside. As such the Delta acts as a regional hydro-refugium. This provides insight, not only to the potential impact of future climate in the region, but also demonstrates why similar basins are attractive to fauna, including our ancestors, in regions like eastern Africa. Although vertebrate evolution operates on time scales longer than decades, the sensitivity of rift wetlands to climate change has been stressed by some authors, and this work demonstrates another example of the unique properties that such basins can afford, given the right hydrological conditions.

Disturbance refugia within mosaics of forest fire, drought, and insect outbreaks

“…) to entire river deltas (Reynolds et al . ). Soil moisture losses from evapotranspiration may be lower in areas of topographic shading and wind sheltering, and soil water‐holding capacity based on soil particle size, organic matter, and bulk density may also help maintain hydrologic refugia (Cartwright ).…”

Springs, palm groves, and the record of early hominins in Africa

“…Groundwater-fed areas are more vegetated than the surroundings; a contrast that is well marked on satellite images (Figs. 2, 3) (Reynolds et al, 2016). The size of a groundwater discharge area varies according to groundwater discharge rate and the geological setting, which may favor the development of a groundwater-fed river or more expansive wetland.…”

Resilience to drought of dryland wetlands threatened by climate change

“…Dryland biomes comprise almost 50% of Earth’s land surface and substantially contribute to global biodiversity and carbon sequestration 1 . In these environments, dryland wetlands are of key importance for regional biodiversity as they serve as habitat sanctuaries for aquatic and terrestrial biota in areas with very few resources 2 , 3 . Periodical flooding events regulate the ecological diversity of the system 4 as flows deliver water, sediment, and associated nutrients to the floodplain 5 , allow fish and invertebrates to reach floodplain environments or distant waterholes 6 , and trigger breeding of waterbirds 3 , 7 and fish 8 .…”